1. Field of the Invention
The present invention relates to a circuit device which improves the heat dissipation of a resin package, and a method of manufacturing the same.
2. Description of the Related Art
As one example of a conventional method of manufacturing a circuit device, the following manufacturing method has been known. As shown in FIG. 9A, a circuit board 71 made of a metal substrate such as an Al substrate is prepared, and an insulative resin layer 72 and a conductive pattern 73 are formed on an upper surface of the circuit board 71. Further, circuit elements 74 and leads 75 are electrically connected to the top of the conductive pattern 73 to form a hybrid integrated circuit on the circuit board 71. Then, the circuit board 71 is placed in a cavity 77 of a resin encapsulation mold 76, and the leads 75 are clamped between an upper mold half 78 and a lower mold half 79. Thus, the circuit board 71 is fixed in the cavity 77.
As shown in FIG. 9B, resin is injected into the cavity 77 through a gate portion 80 of the resin encapsulation mold 76. At this time, as indicated by an arrow 81, the injected resin collides against a side surface of the circuit board 71 first. The resin flows to the upper and lower surface sides of the circuit board 71 as indicated by arrows 81A and 81B. Further, a curved surface 82 is disposed at an edge portion of the lower surface of the circuit board 71, and therefore the resin is allowed to efficiently flow to the lower surface side of the circuit board 71. Although the thickness of a resin encapsulant under the lower surface of the circuit board 71 is, for example, approximately 0.5 mm, the aforementioned resin injecting method enables the narrow gap to be filled with the resin. This technology is described for instance in Japanese Patent Application Publication No. 2003-17515 (pages 6 to 9, and FIGS. 8 and 9).
Moreover, as one example of a conventional circuit device, the following structure has been known. As shown in FIG. 10, in a circuit device 91, a hybrid integrated circuit including a conductive pattern 93 and circuit elements 94 is constructed on an upper surface of a circuit board 92, and the upper, side, and lower surfaces of the circuit board 92 are integrally covered with a resin encapsulant 95. Further, the resin encapsulant 95 includes a first resin encapsulant 95A formed by transfer molding and a second resin encapsulant 95B formed by melting a solid resin sheet. It should be noted that as shown in the drawing, from side surfaces of the resin encapsulant 95, leads 96 electrically connected to the conductive pattern 93 on the upper surface of the circuit board 92 are led out of the resin encapsulant 95. This technology is described for instance in Japanese Patent Application Publication No. 2010-67852 (pages 4 to 10, and FIGS. 1 to 4).
First, in the manufacturing method described with reference to FIGS. 9A and 9B, the resin injected through the gate portion 80 of the resin encapsulation mold 76 collides against a side surface of the circuit board 71, and the utilization of the curved surface 82 formed in the circuit board 71 makes it easy to fill a narrow region under the lower surface of the circuit board 71 with the resin. Further, to prevent the formation of an incompletely filled region in the narrow region under the lower surface of the circuit board 71, a width enough to allow the flow of the resin is needed. Thus, there is the problem that it is difficult to realize a reduction in the thickness of the resin encapsulant under the lower surface of the circuit board 71, and to improve the heat dissipation from the resin encapsulant.
In particular, conceivable ways to solve this problem of heat dissipation include increasing the filler content in resin for encapsulation and increasing the particle size of the filler. However, increasing the filler content or increasing the particle size thereof causes another problem that the flowability of the resin deteriorates to make an incompletely filled region more likely to be formed under the lower surface of the circuit board 71. Furthermore, with regard to the material and shape of the filler used, manufacturing methods in which all the surfaces of the circuit board 71 are integrally encapsulated in a single type of resin also have problems such as damage to circuit elements and breakage of fine metal wires, thus having the problem that the material and shape thereof are limited.
Next, in the circuit device 91 described with reference to FIG. 10, though an incompletely filled region is prevented from being formed under the lower surface of the circuit board 92, and a reduction in the thickness of the resin encapsulant is realized, no disclosure is made with regard to a structure for further improving heat dissipation.
Moreover, a region indicated by a circle 97 represents a polymerized region of the first resin encapsulant 95A and the second resin encapsulant 95B. As shown in the drawing, the second resin encapsulant 95B is formed to extend to the side surfaces of the circuit board 92 to such an extent to cover a lower end portion of the circuit board 92. Further, the polymerized region is prone to have a lower withstand voltage characteristic than single-material regions of the respective first and second resin encapsulants 95A and 95B. Accordingly, there is the problem that the circuit board 92 is short-circuited through the polymerized region when the polymerized region is located under the lower surface of the circuit board 92 or on lower portions of the side surfaces of the circuit board 92.
Further, if a reduction in the thickness of the second resin encapsulant 95B is realized to improve the heat dissipation of the circuit device 91, the percentage of the film thickness of the first resin encapsulant 95A to the thickness of the entire resin encapsulant 95 increases. Accordingly, there is the problem that the circuit board 92 is prone to warpage due to the shrinkage force of first resin encapsulant when the first resin encapsulant 95A is thermally cured in the resin encapsulation mold.